Ultrafiltration membrane and process

ABSTRACT

An inorganic membrane suitable for ultrafiltration or nanofiltration, and methods for making and using the membrane. The membrane has a organic polymer deposited on the feed surface, but is not able to perform separations by solution-diffusion.

This invention was made in part with Government support under contractnumber N00167-03-C-0021, awarded by the Department of the Navy-NavalSurface Warfare Center. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

The invention relates to inorganic ultrafiltration membranes andprocesses. More particularly, the invention relates to inorganicultrafiltration membranes having a polymer treatment that providesfouling resistance.

BACKGROUND OF THE INVENTION

Separation membranes may be made from various inorganic or organicmaterials, including ceramics, metals and polymers. Many membranes areasymmetric in structure, and incorporate a finely porous surface layeror skin and a much more open microporous substrate. The finely poroussurface layer performs the separation; the microporous substrateprovides mechanical strength. Such membranes may be integrallyasymmetric, for example polymer membranes made by the Loeb-Sourirajanprocess, and used for reverse osmosis and gas separation applications,or may be composite structures in which the support and selective layersare formed in separate operations, and made from different materials,for example ceramic composites used for certain ultrafiltration andnanofiltration applications.

Composite membranes that include both inorganic and organic layers havealso been proposed for some types of membrane separation application.

U.S. Pat. No. 3,544,358, to Universal Water Corp., describes a reverseosmosis membrane comprising a cellulosic derivative, such as celluloseacetate, applied as a selective layer onto a porous ceramic support.

U.S. Pat. No. 6,093,325, to Bechtel BWXT Idaho, describes a membrane fordiffusion dialysis, pervaporation or reverse osmosis separations. Themembrane comprises a thin polyphosphazene layer cast onto a porousinorganic or polymer support.

PCT Patent Application WO03/072232 A1, to Creavis Gesellschaft,describes a membrane comprising a ceramic support and an organic polymerselective layer.

U.S. Pat. No. 5,066,398, to Societe Des Ceramiques Techniques, describesa pervaporation membrane comprising a porous inorganic support coatedwith a continuous separating layer of a polyphosphazene. Penetration ofphosphazene into the pores of the support is limited to a depth of lessthan 5 times the pore diameter.

U.S. Pat. No. 5,266,207, to Techsep, describes ananofiltration membranecomprising aporous inorganic support coated with a selective layer of anelastomeric polyphosphazene.

U.S. Pat. No. 4,861,480, to Commissariat a L'Energie Atomique, describesa reverse osmosis membrane comprising an inorganic porous support coatedwith a dense, semipermeable layer of poly(vinylidene fluoride) [PVDF].An ethylenically unsaturated monomer is then grafted onto the PVDFlayer, and the resulting membrane is functionalized to give the layerits separation capabilities.

U.S. Pat. Nos. 5,141,649 and 5,171,449, to Texaco, describepervaporation membranes comprising a non-porous, cross-linked polyvinylalcohol selective layer formed in situ on a porous ceramic support.

U.S. Pat. No. 6,440,309, to Yoram Cohen, describes a pervaporationmembrane comprising a porous ceramic support onto which isgraft-polymerized a vinyl lower alkoxysilane.

U.S. Pat. No. 4,874,516, to NGK Insulators, describes a“semi-ultrafiltration” membrane comprising a porous ceramic supportcoated with a membrane-forming fluorocarbon resin which provides theselective layer and partially permeates the pores of the support.

U.S. Pat. No. 5,342,521, to Commissariat a L'Energie Atomique, describesreverse osmosis or nanofiltration membranes having a porous, inorganicsupport, an intermediate mesoporous metal oxide layer, and a polymericselective layer.

In all of the above cases, the ceramic substrate provides mechanicalstrength and the polymer coating layer is the selective layer thatprovides and governs the separation or rejection properties.

Many different types of polymers may be used for the selective layer ofa membrane. Polyamide-polyether block copolymers have been reported tobe useful as selective layers in polymeric gas separation andultrafiltration membranes, as in U.S. Pat. No. 4,963,165; German patentnumber DE 4237604; an article by K. Ebert et al., “Solvent resistantnanofiltration membranes in edible oil processing,” (MembraneTechnology, No. 107, p. 5-8, 1999); and an article by S. Nunes et al.,“Dense hydrophilic composite membranes for ultrafiltration,” (J.Membrane Science, Vol. 106, p. 49-56, 1995).

Ultrafiltration is a membrane separation process that uses finely porousmembranes to separate water and microsolutes from macromolecules andcolloids. Ultrafiltration membranes operate by permeating water andsmall solutes and rejecting the larger dissolved or suspended materials.A driving force for water permeation is provided by applying an elevatedpressure to the feed liquid or a reduced pressure on the permeate side,or both. At least at low pressure, the water flux through the membraneincreases with increasing pressure difference across the membrane.

Ultrafiltration membranes are very susceptible to fouling. Foulingoccurs when contaminants such as charged solutes, oils, bacteria,colloidal materials of various types, and suspended particulates becometrapped on the surface or in the pores of the membrane. In addition toclogging pores, the accreting material forms a thickening gel layer onthe membrane surface that presents an increasing resistance to waterpermeation. Thus, fouling impairs the membrane performance byprogressively diminishing the transmembrane flux. For a short time, theincreasing resistance presented by the fouling layer can be overcome byincreasing the pressure driving force.

To help control membrane fouling, ultrafiltration systems may bedesigned to include one or more pretreatment steps upstream of theultrafiltration units. These treatments typically include gravityseparation and/or coarser filtration steps to remove potential foulants.In addition, frequent mechanical and/or chemical cleaning procedures arerequired. Although backflushing and chemical cleaning can remove thesurface gel layer reasonably well, they are less successful in removingmaterial trapped inside the membrane pores.

Despite use of the above procedures and operating protocols, foulingcontinues to be a significant problem in at least some applications andreduces the efficiency of many ultrafiltration processes.

There remains a need for intrinsically less fouling ultrafiltrationmembranes. If such a need could be filled, wider applications ofultrafiltration and nanofiltration treatment, such as to industrial andoily wastewaters of many types, or for military or naval use, would bepossible.

SUMMARY OF THE INVENTION

The invention is a membrane suitable for ultrafiltration, includingnanofiltration, and methods for making and using the membrane. Themembrane is highly fouling resistant.

The membrane is made from an inorganic material treated by applying anorganic polymer deposit to the feed surface.

In a basic embodiment, the membrane comprises an inorganic membranecharacterized by the following elements:

-   (a) before being treated as described in element (b) below, the    inorganic membrane has a porous feed surface with an average pore    diameter in the range 10-1,000 Å and is able to function as an    ultrafiltration or nanofiltration membrane;-   (b) the inorganic membrane has been treated by depositing a polymer    region on the porous feed surface;-   (c) after being treated as described in element (b) above, the    inorganic membrane continues to be able to function as an    ultrafiltration or nanofiltration membrane but is unable to function    as a solution-diffusion gas separation membrane.

The structure of the inorganic membrane may be of any type that providesa finely porous feed surface of a porosity appropriate to operating themembrane as an ultrafiltration or nanofiltration membrane. The averagepore diameter on the feed surface is typically and preferably in therange 10-1,000 Å.

Preferably the membrane is a ceramic membrane, such as one made fromalumina or other metal oxide. To achieve a sufficiently finely porousfeed surface, the membrane is preferably a composite inorganic membrane.Membranes of this type are known in the art and may be made by slipcoating or by a sol-gel process, for example. The membrane may be in theform of sheets, tubes, monolithic blocks, capillary fibers or any otherconvenient form.

After the inorganic membrane has been formed, it has a surface porosityand pore size such that it could be operated without the organic polymertreatment as an ultrafiltration or nanofiltration membrane.

The membrane is treated by applying a very thin coat of a dilute organicpolymer solution. This deposit covers or partially covers most of thepore openings.

Unlike coatings that provide the selectivity or separation capability ofa membrane, the polymer deposit should not form a dense, continuous,gas-tight layer. If such a layer were to be deposited, it would reducethe water flux of the membrane to an unacceptable level.

Rather, the deposit should be extremely thin, typically no more thanbetween about 0.1 μm and about 0.5 μm thick, and may be discontinuous,so that pinholes, cracks or regions of uncoated inorganic membranesurface remain. Thus, the treatment creates one or multiple regions ofpolymer deposit on the feed surface, but the deposit is so thin or leakythat the transmembrane water flux remains relatively high.

The thinness or leakiness of the polymer deposit means that, unlike theprior art membranes described above, the polymer deposit does notcontrol the separation properties. Thus, the treated membrane could notperform separations that require a membrane with a dense, essentiallycontinuous and defect-free polymer layer, such as reverse osmosis,pervaporation or gas separation.

Using gas separation as an example of these types of separation,permeating gases dissolve in the polymer material of the dense layer,then diffuse through it. This mechanism is called solution-diffusion andmembranes that separate on this principle are called solution-diffusionmembranes. The selectivity is determined by the solubility anddiffusivity of the gases in the polymer. This selectivity is known formany polymers and gas pairs, and can be measured by simple permeationexperiments.

The membranes of the invention are unable to operate in this manner.They exhibit a gas selectivity that is inconsistent withsolution-diffusion as the dominant transport mechanism, as explained inmore detail below.

The organic polymer used to treat the membranes is preferablyhydrophilic, and most preferably is water swellable but water insoluble.When the polymer is water insoluble, crosslinking may not be requiredafter the polymer has been deposited. Preferably the polymer is rubbery,or at least has a rubbery segment.

The treatment is not relied upon to impart ultrafiltration ornanofiltration capability. In general, the rejection properties of thetreated membranes are in a similar range to those of an untreatedmembrane of the same structure and composition.

The treatment typically results in a membrane that exhibits a lowerinitial water flux than the initial water flux of an untreated membraneof the same structure and composition, if the treated and untreatedmembranes are challenged with the same feed solution under the sameconditions.

However, the treatment also results in membranes that are much lesssusceptible to fouling than their untreated equivalents when challengedwith common contaminants such as oils, or other organic compounds. Forexample, flux decline, or resistance increase, over a period of hours,days or weeks for the treated membranes may be half or less than wouldoccur with untreated membranes of the same structure and composition.Internal fouling, caused by material trapped inside membrane pores orsurface crevices, is believed to be nearly eliminated by the treatment.

The invention further includes methods of making the treated membrane.In this aspect, the invention comprises the following steps:

-   (a) manufacturing or otherwise providing an inorganic membrane    having a feed surface with an average pore diameter in the range    about 10-1,000 Å;-   (b) preparing a solution of a polymer having a polymer concentration    of up to about 0.5 wt % polymer in a solvent;-   (c) coating the solution onto the feed surface;-   (d) allowing the solvent to evaporate, thereby forming a polymer    region on the feed surface; the method being characterized in that    after steps (b) and (c) have been performed the inorganic membrane    is unable to function as a solution-diffusion gas separation    membrane.

The membranes may be made by preparing an inorganic ultrafiltration ornanofiltration membrane according to any convenient technique, thenapplying the organic polymer treatment as a manufacturing step. Thetreatment may also be used to improve the properties of already madecommercial membranes, preferably before they are used for the firsttime.

The polymer solution applied to the feed surface is very dilute, such asno more than about 0.5 wt % polymer, preferably no more than about 0.3wt % polymer, and most preferably no more than about 0.2 wt % polymer oreven 0.1 wt % polymer. The solution is applied to the feed surface usingany solution coating method familiar in the art. This may be as simpleas pouring the solution over the membranes, for example. The membrane isthen dried, leaving a polymer deposit on the feed surface.

The invention also includes methods of using the treated membrane. Inthis aspect, the invention comprises the following steps:

-   (a) providing an inorganic membrane characterized by the following    elements:-   (i) before being treated as described in element (ii) below, the    inorganic membrane has a porous surface with an average pore    diameter in the range 10-1,000 Å and is able to function as an    ultrafiltration or nanofiltration membrane;-   (ii) the inorganic membrane has been treated by depositing a polymer    region on the porous surface;-   (iii) after being treated as described in element (ii) above, the    inorganic membrane continues to be able to function as an    ultrafiltration or nanofiltration membrane but is unable to function    as a solution-diffusion gas separation membrane;-   (b) passing a feed stream containing a contaminant removable by    ultrafiltration across the feed side under filtration conditions;-   (c) removing from the permeate side a treated water stream depleted    in the contaminant.

The process may be carried out in any mode and may include additionalsteps as required, including, but not limited to, pretreatment orfollow-on treatment, or more than one membrane separation step.

Contaminants that may be removed by the process of the invention includedissolved organic materials, emulsions, colloids and suspended materialsincluding oils, grease, bacteria and particulates.

In general, the tolerance of the processes of the invention for handlingstreams that contain significant quantities of solutes and undissolvedmatter is higher than that of prior art processes. In somecircumstances, this means that a lesser degree of pretreatment isneeded, or the process can be operated for a longer time before membranecleaning is needed, or the process may be used to treat a stream thatwas previously untreatable, either on technical or economic grounds, byultrafiltration.

The invention is expected to be useful in all areas where inorganicultrafiltration membranes can be used, including, but not limited tofood and beverage processing, process water treatment in thebiotechnology, chemical and pharmaceutical industries, wastewatertreatment, and municipal water treatment. The invention is expected tobe particularly useful in treating oily wastewaters.

Other objects and advantages of the invention will be apparent from thedescription of the invention to those of ordinary skill in the art.

It is to be understood that the above summary and the following detaileddescription are intended to explain and illustrate the invention withoutrestricting its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing a cross-section of a treatedultrafiltration membrane.

FIG. 2 is a graph showing water flux as a function of operating time foruntreated and treated elements exposed to 2,000 ppm and 5,000 ppmoil/water emulsions.

FIG. 3 is a graph showing water flux as a function of operating time foruntreated and treated elements exposed to 2,000 ppm and 5,000 ppmoil/water emulsions, and to a synthetic bilgewater mixture.

FIG. 4 is an SEM photo showing a treated composite ceramic membrane,showing the silicone carbide support layer and the silica selectivelayer.

FIG. 5 is an SEM photo showing only the selective layer of the membraneof FIG. 4. The PEBAX deposit is too thin to be observed at amagnification of 800. The particles on the surface are residual gritfrom various feed solutions used in the permeation tests.

FIG. 6 is a graph showing results of a long-term comparative permeationtest carried out with treated and untreated lab-scale ceramic elements.

FIG. 7 is a graph showing results of a long-term permeation test with acommercial-scale treated ceramic element.

FIG. 8 is a schematic diagram of a basic embodiment of the process ofthe invention.

FIG. 9 is a schematic diagram of a preferred embodiment of the processof the invention operated in feed-and-bleed mode.

DETAILED DESCRIPTION OF THE INVENTION

All percentages herein are by weight unless otherwise stated.

The term ultrafiltration includes nanofiltration.

The term selectivity as used herein to refer to gas separation means theselectivity measured using individual pure gases.

The invention is a membrane suitable for ultrafiltration, includingnanofiltration, and methods for making and using the membrane. Themembrane is highly fouling resistant.

In a first aspect, the invention is an ultrafiltration membrane,indicated generally as 1 in FIG. 1. The membrane includes at least twoelements: an inorganic ultrafiltration membrane, 2, and an organicpolymer deposit, 3.

The structure of inorganic membrane 2 may be of any type that provides afinely porous feed surface of a porosity appropriate to operating themembrane as an ultrafiltration or nanofiltration membrane.

Various types of inorganic membranes are known, including those madefrom metals, carbon, glass, inorganic polymers and ceramics. Themembrane can be made from any of these, so long as it can function as anultrafiltration membrane. Inorganic membranes suitable for use asultrafiltration membranes are described in Ultrafiltration andMicrofiltration Handbook, by Munir Cheryan, Technomic PublishingCompany, Lancaster, Pa., 1998. Preferably the membrane is a ceramicmembrane.

To achieve a sufficiently finely porous feed surface, the inorganicmembrane is usually a composite structure, comprising a porous supportthat has been coated with one or more layers having a smaller pore sizethan the support. If multiple coatings have been applied, the pore sizeusually decreases with each layer, with the outermost, most finelyporous layer determining the filtration properties.

The feed surface, that is, the surface that will be exposed to theliquid to be treated, should have an average pore size of between about10 Å and 1,000 Å.

The membrane may be in the form of sheets, tubes, perforated blocks,capillary fibers or any other convenient form known in the art andappropriate to the intended use.

A preferred membrane is a ceramic membrane and preferred forms areceramic tubes and monoliths with internal tubes. Both types of ceramicmembrane are manufactured by several commercial suppliers and arerelatively easy to handle and use.

The untreated inorganic membrane should provide a high pure water flux,since the flux will be reduced by the polymer treatment. Preferably theinorganic membrane should provide a water flux, measured with a new,clean membrane at 25° C. and a feed-to-permeate transmembrane pressuredifference of 50 psi of at least 100 gfd (gallons per square foot perday), more preferably at least 150 gfd and most preferably at least 200gfd.

The membrane is treated by applying a thin coat of a dilute organicpolymer solution to the feed surface of the membrane. After the solventhas evaporated, an extremely thin polymer deposit, 3, is left on thesurface, and this deposit covers or partially covers at least some ofthe pore openings. This deposit is so thin that its thickness cannot bemeasured easily by any direct measurement technique, nor can it bereadily distinguished as a discrete coating layer even by scanningelectron microscopy at high magnification.

The deposit is different in both structure and performance from thepolymer coatings described in the prior art discussed above. Unlikethese coatings, the polymer deposit does not provide the selectivity orseparation capability of the membrane, and does not form a dense,continuous, gas-tight layer. If such a layer were to be deposited, itwould reduce the water flux of the membrane to an unacceptable level.

Rather, deposit 3 should be extremely thin, typically less than 0.5 μmthick, and preferably no more than about 0.2 μm thick or most preferablyno more than about 0.1 μm thick. Further, the deposit may be, andtypically is, discontinuous, so that at least one or more pinholes,cracks, or small regions of uncoated inorganic membrane surface remain.Thus, the treatment creates one or multiple regions of polymer depositon the feed surface, but the deposit is so thin or broken (and henceleaky) that the transmembrane water flux remains adequately high.

The thinness or leakiness of the polymer deposit means that, unlike theprior art membranes described above, the polymer deposit does notcontrol the separation properties. A measure of this leakiness is thatthe treated membrane could not be relied on to perform adequately forseparations that require a dense membrane with relatively few or nodefects, such as reverse osmosis, pervaporation or gas separation.

Without wishing to be bound by theory, this desired attribute of thedeposit can be conveniently quantified with reference to gas separationprinciples.

Gas permeation through a polymeric membrane may take place in severalways. If the membrane is porous and the pores are large—above about 0.1μm—gases permeate the membrane indiscriminately by convective flow, andno separation occurs.

If the pores are smaller than about 0.1 μm (1,000 Å), the average porediameter is as small or smaller than the mean free path of the gasmolecules. (At atmospheric pressure the mean free path of common gasesis in the range 500 to 2,000 Å.) Transport through such pores isgoverned by Knudsen diffusion, and the selectivity of a gas pair isinversely proportional to the square root of the ratios of the molecularweights of the gases. Unless the gases are of very different molecularweight, therefore, their transport rates are similar and the membraneexhibits low selectivity in favor of one gas over another.

An untreated ultrafiltration membrane has an average surface pore sizeconsistent with Knudsen diffusion and, if tested for its gas separationproperties, typically exhibits separation characteristics at leastapproximately consistent with this transport mechanism. For the gas paircarbon dioxide/nitrogen, for example, the selectivity is on the order of√{square root over ( )}(28/44), or 0.8 for carbon dioxide over nitrogen,and √{square root over ( )}(44/28), or 1.3 for nitrogen over carbondioxide. That is, an ultrafiltration membrane offers only slightselectivity between the gases, and does so in favor of nitrogen overcarbon dioxide.

Likewise, for the gas pair oxygen/nitrogen, the selectivity of anuntreated ultrafiltration membrane is on the order √{square root over ()}(28/32), or 0.9 for oxygen over nitrogen and √{square root over ()}(32/28), or 1.1 for nitrogen over oxygen. Again, the ultrafiltrationmembrane offers only marginal selectivity between the gases, in favor ofnitrogen over oxygen.

When the pore diameter of a membrane decreases to the 5 to 10 Å range,the pores begin to separate gases by a molecular sieving effect, andhigh selectivity is possible in principle.

Ultimately, if a polymeric membrane is without permanent pores, butforms a dense polymer layer, the transport mechanism is no longer bypore flow. Instead, permeant gases dissolve in the polymer material anddiffuse through the membrane down a concentration gradient. Individualpermeants are separated because of the difference in their solubilitiesin the polymer material and the difference in the rates at which theydiffuse through it. A defect-free, dense polymer film will exhibit aselectivity in favor of one gas over another that represents thissolubility-diffusivity driven selectivity through the polymer itself.

This selectivity is known for many polymers and gas pairs, and can bemeasured by simple experiments. For example, all polymeric membranesseparating gases by solution-diffusion exhibit selectivity in favor ofoxygen over nitrogen. For an average polymer, this selectivity isgenerally in the range between about 2 and 7, compared with 0.9(nitrogen selective) for the selectivity based on Knudsen diffusionselectivity.

Similarly, all polymers exhibit solution-diffusion selectivity in favorof carbon dioxide over nitrogen, with typical selectivities rangingbetween about 5 and 50 or more, compared with 0.8 (nitrogen selective)for the selectivity based on Knudsen diffusion.

Gas separation membranes are much more sensitive to defects, such aspinholes or cracks in the selective membrane layer, than membranes usedin ultrafiltration. For example, if a defect-free polymeric membrane hasa selectivity of 100 for hydrogen over nitrogen, a correspondingmembrane with one or two widely spaced pinholes may well have aselectivity less than 50, only half the value of the perfect membrane.If larger discontinuities, such as more numerous pinholes, or one ormore cracks or breaks in the membrane layer are present, the gasseparation capability of the selective layer is typically lostcompletely, and the membrane exhibits the gas separation properties, ifany, of the underlying microporous substrate.

The relevance of the above discussion to the invention is that thecomparative gas separation properties of the untreated and treatedmembranes can be used to indicate whether and to what extent the polymertreatment has resulted in a continuous coating of the membrane feedsurface.

If the treatment has yielded a perfect, or close to perfect coating, themembrane will be fairly gas-tight and will exhibit a selectivityapproaching the selectivity of the polymer. Such a membrane is outsidethe scope of the invention. If the treatment has left an ultrathin,broken or discontinuous deposit, the gas separation selectivity will beat or close to the Knudsen diffusion selectivity of the inorganicultrafiltration membrane. If the treatment has left a deposit with justa very few defects, the selectivity will be somewhere between theKnudsen diffusion and solution-diffusion values.

Because the polymer deposit cannot be readily seen or its thicknessmeasured, the gas separation performance provides a simple quantitativetest of the membrane characteristics. We prefer to make this measurementwith common gases, such as oxygen and nitrogen, or carbon dioxide andnitrogen.

We regard the polymer treatment as meeting our requirements if themembrane gas selectivity is substantially less than thesolution-diffusion selectivity of the applied polymer. By substantiallyless, we mean that the membrane gas selectivity is less than half thesolution-diffusion selectivity of the applied polymer.

More preferably, expressed in numerical terms, the membrane gasselectivity should be no more than about 1.3 for oxygen over nitrogen,and no more than about 2 or 3 for carbon dioxide over nitrogen.

Most preferably, the membrane should have no significant gasselectivity, such that gas transport remains substantially close to theKnudsen diffusion value.

The organic polymer used to treat the membranes should preferably havecertain properties. First, the polymer should preferably be hydrophilic,so as not to impede water transport through the membrane and todiscourage adhesion of hydrophobic fouling material to the membranesurface.

In addition, the polymer should preferably be water swellable but waterinsoluble. By water insoluble, we mean that the polymer will notdissolve to any meaningful concentration in liquid water at anytemperature. By meaningful concentration, we mean a concentration abovethe tens of ppm level.

By water swellable, we mean that on immersion in liquid water for aprolonged period, such as 24 hours, the weight of a polymer sampleincreases by a measurable amount, such as 10 wt % or more. Preferablythe water sorption should be substantially higher than this, so that thepolymer swells by a weight increase of 40% or more, and more preferablyby 70% or more, and most preferably by at least 100%.

The most preferred polymers should thus swell extensively in waterwithout dissolving.

A particular advantage of using a water-insoluble polymer for thetreatment is that crosslinking may not be required after the polymer hasbeen deposited to render it stable. This simplifies the membranepreparation technique.

In the alternative, water-soluble polymers may be used and may becrosslinked after deposition to prevent the polymer gradually washingoff when the membrane is used to treat aqueous solutions.

The polymer may be glassy or rubbery. Many water-swellable orwater-soluble polymers are known and their properties will be familiarto those of skill in the art.

Glassy polymers that may be used include, but are not limited to,polyvinyl alcohol (PVA), cellulose derivatives, acrylic-based polymersand polyvinylpyrrolidone. When in a water-swollen state, these materialsoffer relatively high water permeability.

In general, however, we prefer to use polymers that are rubbery, or atleast include a rubbery segment within the polymer structure, becauserubbery polymers tend to provide higher intrinsic water permeabilitythan glassy polymers. By a rubbery polymer, we mean a polymer that isabove its glass transition temperature under the conditions at which themembrane is likely to be operated. Thus, any polymer that has a glasstransition temperature below 0° C. meets this definition, and ingeneral, any polymer that has a glass transition temperature below about10° C. meets this definition.

Preferred polymers that are rubbery or contain rubbery segmentssufficient to meet the above definition include epichlorohydrinpolymers; epichlorohydrin copolymers, such as epichlorohydrin-ethyleneoxide copolymers; polyethylene oxide polymers and copolymers; allylglycidyl ether polymers; polyurethanes; and other copolymers havingrelatively high proportions of polyether blocks, by which we meanpreferably 50% polyether or higher.

Most preferably, the polymer is a polyamide-polyether block copolymer.Such polymers provide high intrinsic permeability to water, and we havediscovered that they can be prepared and coated well onto inorganicultrafiltration membranes, even from extremely dilute solutions.

Such polymers are available commercially, for example from ATOFINAChemicals, Philadelphia, Pa. under the name PEBAX®. PEBAX polymers havethe general formula

where PA is a polyamide segment, PE is a polyether segment and n is apositive integer. The polyamide blocks provide mechanical strength; thepolyether blocks allow high water permeability.

These polymers are available in a range of grades having differentproportions of polyamide and polyether. Preferred grades in terms ofwater sorption capabilities are grade 1074, which has the followingstructure:

and grade 1657 which has the following structure:

For preference, the treated membrane should present a surface to thefeed solution that is electrically neutral overall, which means that thepolymer used for the treatment should be electrically neutral. Morespecifically, polymers having ion-exchange properties or containing freecharged groups, such as the sulfonated polymers described in U.S. Pat.No. 6,026,968, should be avoided.

The treated ultrafiltration membrane exhibits a number of distinctiveproperties, when compared under like conditions with an otherwiseidentical untreated membrane.

First, the treatment is not relied upon to impart ultrafiltrationseparation properties. The untreated membrane to which the treatment isapplied is already capable of functioning as an ultrafiltration ornanofiltration membrane.

The treatment slightly increases the thickness of the membrane overall,as well as at least partially covering the pores, so even if the polymerthat is used has very high intrinsic water permeability, the treatedmembrane typically exhibits a lower initial water flux and higherpermeation resistance than the initial water flux and resistance of anuntreated membrane of the same structure and composition. The initialwater flux is the flux measured with a clean, previously unused membraneas originally supplied by the commercial manufacturer or as preparedaccording to the teachings herein.

For example, the initial water flux, when tested with pure water, and atthe same temperature and pressure, may be 250 gfd for an untreatedsample and may drop to 50 gfd for a treated sample. As another example,the initial water flux, when tested with a 2,000-ppm motor oil emulsion,may be 300 gfd for an untreated sample and 80 gfd for a treated sample,both measurements being made at the same temperature and pressure. Thus,the treatment may reduce the initial water flux substantially, such by afactor of 2, 4, 6 or more.

The higher the intrinsic water permeability of the treating polymer, andthe thinner the deposit, the less will the initial water flux bereduced. On the other hand, if the treatment results in polymer depositsthat are so insubstantial that many fully open pores remain exposed orlarge regions of the feed surface remain untreated, the ability of themembrane to resist fouling will not be significantly improved.

The chosen balance between initial flux reduction and fouling resistancewill vary depending on the specifics of the use to which the membrane isput, as well as the magnitude of the untreated membrane flux. Ingeneral, a pure water flux reduction after treatment to as low as 20% oreven less of the untreated initial value can be acceptable, so long asthe initial flux after treatment remains adequate.

In absolute terms, it is preferred that the treated membrane provide aninitial pure water flux under typical ultrafiltration conditions, suchas 50 psi feed-to-permeate transmembrane pressure difference and 25° C.,of at least about 40 gfd, more preferably at least about 80 gfd and mostpreferably at least about 100 gfd.

Although their initial water fluxes tend to be low compared with priorart membranes, in use our treated membranes are much less susceptible tofouling than their untreated equivalents. That is, when they are exposedto common contaminants such as oil, grease, bacteria or particulates,the treated membranes manifest a much slower rate of flux decline thanthe untreated membranes.

In a second aspect, the invention includes methods for making thefouling-resistant membranes described above. The polymer treatment maybe applied as a step when the membrane is being manufactured, or may beapplied to improve the properties of already made commercial membranes,preferably before they are used for the first time.

The preparation of inorganic ultrafiltration membranes is welldocumented in the literature, and is described, for example, in manypatents, including U.S. Pat. Nos. 4,983,423; 5,106,502; and 5,824,220.Extensive discussion of preparation techniques is given in variouspapers and books, such as Inorganic Membranes; Synthesis,Characterization and Applications, by R. R. Bhave (ed), Chapman Hall,New York, 1991.

As already mentioned, preferred membranes are composite ceramicmembranes. Membranes of this type can be made by applying one or severalsuccessive ceramic coatings to a porous support.

Suitable materials for the porous support include refractory oxides,such as alumina, zirconia, cordierite (magnesium aluminum silicate),mullite (an aluminosilicate) and spinel (magnesium aluminum oxide),carbon, sintered steel or other metals, and carbides, such as siliconcarbide.

Commonly such supports are manufactured by preparing a paste ofparticles with binders, viscosity modifiers, lubricants, wetting agentsand the like as required. The paste is poured into a mold, extrudedthrough a die or otherwise shaped to the desired geometry, baked toremove organic components, then sintered at very high temperature tocreate the finished porous article. After it has been finished, thesupport typically has pores of about 1-10 μm in diameter.

The support may be of any convenient shape for subsequent coating.Ceramic tubes and monolithic blocks perforated by parallel tubes to forma “honeycomb” cross section are both readily available commercially.

To form the ultrafiltration selective layer, at least one surface of thesupport must be coated with one or multiple layers having finer poresthan the support. The coating may be deposited in any way, byslip-casting or dynamic filtration, for example. The coating ispreferably deposited by slip-casting to make a more open ultrafiltrationmembrane, and preferably by the sol-gel technique to make a tighterultrafiltration or nanofiltration membrane. The two processes may alsobe used in succession to make a tighter membrane.

Suitable materials for the selective layers of the ultrafiltrationmembrane include the refractory oxides mentioned above, as well ascarbides and nitrides, for example. To slip-cast this layer onto thesupport, a suspension of particles of the desired material, withbinders, viscosity modifiers or other agents as needed is prepared. Theslip is brought into contact with the support for a few seconds, byfilling the tubes or cavities with slip, or simply by pouring it overthe surface to be coated. The excess slip is drained off, and the coatedform is dried and fired. The temperature and duration of the firingstep(s) determine the final pore size and porosity.

Usually several slip-coated layers are applied in series, each layerbeing formed from a suspension of progressively finer particles. Theslip coating-sintering procedure can be used to make membranes with porediameters down to about 100 to 200 Å.

If more finely porous membranes are required, they may be made by asol-gel technique. Such techniques for preparing ceramic materials withvery fine pores are known in the art and are described in U.S. Pat. No.3,944,658, for example. Methods of making membranes with very fine poresusing these techniques are described in U.S. Pat. Nos. 5,006,248 and5,208,190, for example.

In brief, the membranes may be made by forming a sol of fine metalalkoxide particles, preferably alumina. This is preferably done byadding the alkoxide to water that is being rapidly stirred or agitated.A hydrolysis reaction takes place, forming metal hydroxide, whichprecipitates into the water in the form of nanoparticle-sized clustersof metal hydroxide monomer or dimer particles, or larger clusters ofpolymeric particles, depending on the amount of water used. Acid isadded and the solution is maintained at an elevated temperature for anextended period to stabilize the sol by keeping the particles insuspension. After cooling, a stabilized colloidal sol remains.

The sol can be coated in this form onto any appropriately microporoussupport. Coating is preferably carried out by dipping or spraying. Thesupport is then dried carefully under controlled conditions to avoidcracking, converting the coating into a gel. Alternatively, the sol maybe partially gelled into a relatively viscous state before coating, andadditives such as polymeric binders may be included to control viscosityand surface tension.

Finally the coated support is sintered at high temperature to form thefinished membrane. Membranes with pore sizes in the 10- to 100-Å rangecan be made by this method.

As a preferred alternative to manufacturing the inorganic membranes,they may be purchased from commercial suppliers. Corning Incorporated,of New York, offers ceramic ultrafiltration membranes in the form of ahoneycomb ceramic monolith or as alumina tubes. Ceramem Corporation, ofWaltham, Mass., offers ultrafiltration and nanofiltration membranes astubular monoliths of silica or titania on a recrystallized siliconcarbide support.

Whether the inorganic membrane is manufactured from raw materials orpurchased, the making of the membrane is completed by applying thepolymer treatment to the membrane feed surface. This can be done by anysuitable technique, such as dipping or spraying. Since the feed surfaceis usually the inside surface of the tubes or monolith, convenientmethods are simply to pour the solution over these surfaces, or to filland drain the tubes or blocks.

The coating solution should be very dilute, such as less than 0.5 wt %polymer and preferably less than 0.3 wt % polymer. Our most preferredsolutions contain only 0.1 wt % of polymer. The inorganic membranes haveexcellent chemical resistance to attack by organic liquids, so anyorganic solvent that will dissolve the chosen polymer may be used. Ifthe most preferred polyamide-polyether block copolymers are used as thecoating material the most preferred solvent is n-butanol or otheraliphatic alcohol.

The membranes are drained and dried, leaving a thin polymer deposit onthe surface. The process may be repeated to provide a slightly thickeror more complete coating. However, even two applications of 0.2 wt %solution may result in a membrane with too low initial water flux, so weprefer to apply the treatment only once.

After evaporation of the solvent, the polymer deposit left on themembrane is so thin that it cannot be seen as a discrete layer by SEM ata magnification of 800. We believe the deposit is less than 0.5 μmthick, and likely only about 0.1 μm thick.

In a third aspect, the invention includes ultrafiltration andnanofiltration processes using the treated inorganic membranes. Theseprocesses can be applied to treatment of aqueous or organic solutions.To simplify the description, processes are described below in terms ofwater treatment. It will be appreciated by those of skill in the artthat similar methods and modes of operation can be used to treatsolutions in which the solvent is organic. Organic solutions that may betreated by the process of the invention include those in which thesolvent is an alcohol, such as ethanol, an alkane, such as hexane, or anaromatic, such as toluene.

Contaminants that may be removed by the process of the inventionencompass all of those within the scope of conventional ultrafiltrationand nanofiltration. These, include, but are not limited to, emulsifiedor colloidal organic matter, such as greases, oils or paints; bacteriaand viruses; dissolved macromolecules, such as proteins, dyes, ordetergents; and moving toward the nanofiltration end of the scale,dissolved or colloidal lighter organic materials, such as lightweightoils, sugars, and multivalent salts, with molecular weights down toabout 70.

In its most basic form, as shown in FIG. 8, the process includes passinga feed stream, 81, into a membrane module, 82, containing a treatedinorganic membrane, 83, in accordance with the description above. Thefeed stream flows across the feed side, 84.

A driving force for transmembrane permeation is provided by a pressuredifference between the feed and permeate sides of the membrane. Thispressure difference can be achieved by applying a higher thanatmospheric pressure on the feed side, a lower than atmospheric pressureon the permeate side, or a combination of both. This pressure differenceis typically up to about 200 psi for nanofiltration, and preferablylower for ultrafiltration, such as no higher than about 100 psi, 70 psi,60 psi, 50 psi or less. The pressure difference may be held constant, ormay be gradually increased to overcome resistance and hold flux constantif fouling occurs.

For some applications, such as shipboard applications, operation atconstant flux is preferred, by maintaining a fixed feed pressure andgradually lowering the permeate pressure.

Under this driving force, water permeates the membrane and is withdrawnas treated permeate or filtrate stream, 86. Depending on the grade ofmembrane used, material down to a diameter of about 10 Å can be rejectedby the membranes with rejections of 90% or better. The residue orretentate stream, 85, concentrated in matter that has been rejected bythe membrane, is withdrawn from the residue end of the modules on thefeed side.

The process may be carried out in any mode, including single pass,batch, and feed-and-bleed modes, all of which are familiar in theindustry.

A preferred mode is feed-and-bleed, a representative, simple embodimentof which is shown in FIG. 9. It will be appreciated by those of skill inthe art that this is a very simple schematic diagram, intended to makeclear certain aspects of the invention, and that an actual process mayinclude additional components of a standard type, such as heaters,chillers, pumps, blowers, other types of separation and/or fractionationequipment, valves, switches, controllers, pressure-, temperature, level-and flow-measuring devices and the like.

Turning now to FIG. 9, raw feed stream, 91, enters the process throughfeed pump 92 and passes into optional pretreatment step 93. This stepprepares the raw stream for ultrafiltration by removing grossundissolved matter.

Any suitable type of treatment can be used for this step, includinggravity separation and filtration. The coarsest particulates can beremoved simply by running the feed through settling tanks or parallelplate separators, for example. Less coarse particulates may be removedby cyclone separation, by a low- or high-speed centrifuge, or bydead-end cake filtration. To remove smaller particles, less than about 5μm in diameter down to about 0.1 μm in diameter, microfiltration can beused.

If the source of the feed is such that suspended matter with a range ofparticle sizes is present, step 93 may include a train of individualsteps, for example separation in a parallel plate separator, followed byone or two microfiltration steps in series. Such treatments are veryfamiliar to those of skill in the art.

Pretreated stream 94 enters the feed-and-bleed processing loop,generally indicated as 96, where it is mixed with recirculating stream104 to form feed stream 97 to ultrafiltration unit 98. Circulation ofliquid in the processing loop is provided by circulation pump, 95.

To provide the driving force for transmembrane permeation, an elevatedpressure on the feed liquid may be provided by feed pump 92, or areduced pressure on the permeate side by pump 107, or both.

The ultrafiltration unit 98 is equipped with one or more membranemodules containing ultrafiltration membranes, 99, as described above,having feed surfaces, 100. The modules can be arranged in parallel orseries arrangements, both of which configurations are well known andcommon in the art. The stream under treatment circulates across the feedsurfaces of the membranes and exits the residue ends of the modules asstream 101, passes through open valve 102 and is recirculated as stream104 in the processing loop. Treated concentrate stream, 105, is drawnoff as desired by opening valve 103.

Under the influence of the transmembrane driving force, the water orother liquid being filtered permeates the membranes and is withdrawn aspermeate stream 106 by permeate pump 107.

The process may include any additional steps as required. For example,either the concentrated retentate stream or the permeate stream, orboth, may be sent to further treatment, including, but not limited to,other membrane separation steps, such as additional ultrafiltrationsteps or nanofiltration or reverse osmosis steps.

As one non-limiting example, it is common practice to send the retentateto a second feed-and-bleed step, to send the retentate from that step toa third feed-and-bleed step and so on, for a typical series of three,four or five steps. By splitting the separation into multiple steps, thedegree of retentate concentration in each step is limited, and higherflux is maintained in each step.

If required, more details of various operating modes, and the effect oftheir operating parameters on processing results, may be found inChapter 7 of Ultrafiltration and Microfiltration Handbook, by MunirCheryan, Technomic Publishing Company, Lancaster, Pa., 1998.

A feature of the membranes and processes of the invention is theirability to resist fouling. As illustrated in the Examples section below,when operated at constant feed and permeate pressure, the membranes ofthe invention are often able to maintain flux at or close to the initialvalue for periods of days or weeks without cleaning, even whenprocessing oil emulsions or comparatively highly fouling mixtures, suchas bilgewater. Expressed another way, the resistance to permeationincreases relatively little over time, so that an increased drivingforce is not required to maintain flux.

Although the membranes of the invention are less susceptible to foulingthan prior art membranes, they may be cleaned by backflushing from timeto time as needed, or by using chemical cleaners. It is expected thatintervals between cleaning may be longer than were previously neededwhen handling a comparable feed. For example, if a membrane systemhandling a wastewater stream would have needed daily backflushing ifprior art membranes were used, it may be possible to operate the systemwith weekly cleanings using the treated inorganic membranes of theinvention.

Internal fouling, caused by material trapped inside membrane pores orsurface crevices, is believed to be nearly eliminated by the organicpolymer treatment. As a result, the membranes of the invention oftenrecover their flux much better after cleaning than their untreatedcounterparts, as illustrated in the Examples section below.

Application areas include the existing large applications of treatmentof a variety of industrial oily wastewaters. In addition, feeds thathave previously been more difficult to treat by ultrafiltration, such asmunicipal wastewaters and industrial wastewaters containing multiplecontaminants are more easily treated using the lower fouling, treatedmembranes of the invention.

A number of specialized applications are also possible, such astreatment of produced water in oil and gas fields, wastewaters fromships, production of potable water on ships, and treatment of littoralwater for a variety of uses.

The invention is now further described by the following examples, whichare intended to be illustrative of the invention, but are not intendedto limit the scope or underlying principles in any way.

EXAMPLES Example 1 Untreated Membrane Pure-Gas Permeation Properties

A number of specialized applications are also possible, such astreatment of produced water in oil and gas fields, wastewaters fromships, production of potable water on ships, and treatment of littoralwater for a variety of uses.

The invention is now further described by the following examples, whichare intended to be illustrative of the invention, but are not intendedto limit the scope or underlying principles in any way. A number ofspecialized applications are also possible, such as treatment ofproduced water in oil and gas fields, wastewaters from ships, productionof potable water on ships, and treatment of littoral water for a varietyof uses.

Three ceramic ultrafiltration membrane elements (Corning, Inc., Corning,N.Y.) were obtained. The elements were tubular monoliths of a silicaselective layer on a mullite (alumina silica oxide) support. Theelements were 1 inch in diameter, 12 inches long, and had an effectivemembrane area of 0.14 m². The nominal pore size of the elements was 5 nm(50 Å).

The membrane elements were tested in a bench-scale, stainless steel,ultrafiltration test system with pure oxygen, nitrogen, and carbondioxide. The feed pressure was 1 psig, the permeate pressure wasatmospheric, and the temperature was 22° C. Because the membranes wereexpected to have very high gas fluxes, the feed pressure was kept verylow, to keep the fluxes within measurable limits. The fluxes weremeasured and the permeances (pressure-normalized fluxes) werecalculated. The results are shown in Table 1.

TABLE 1 Permeance Membrane (10⁻⁶ cm³(STP)/cm² · s · cmHg) (gpu) ElementCarbon Dioxide Nitrogen Oxygen #1 10,100 10,100 10,100 #2 10,100 10,10010,100 #3 10,600 9,040 9,040

As can be seen, Elements #1 and #2 showed no selectivity between any ofthe gases within the accuracy of the experiment (±about 1,000 gpu atthis high flux). Element #3 showed slight selectivity (1.2) in favor ofcarbon dioxide over nitrogen. This very slight selectivity in favor ofcarbon dioxide may arise from the limits of experimental accuracy, ormay indicate that the pore size of the membranes is so small that someeffects of molecular sieving are beginning to be seen.

The results for all three elements are inconsistent with gas separationby solution-diffusion.

Example 2 Treated Membrane Pure-Gas Permeation Properties

Elements #1 and #2 from Example 1 were treated with apolyether-polyamide block copolymer (PEBAX®, Atofina Chemicals,Philadelphia, Pa.). A solution of 0.5% PEBAX 1074 in n-butanol waspoured through the ceramic elements. The elements were allowed to dryand a second treatment was applied. The elements were tested with puregases as in Example 1; the feed pressure was increased to 10 psig,however. The results are shown in Table 2.

TABLE 2 Mem- Permeance (gpu) brane Carbon Dioxide Nitrogen OxygenElement Untreated Treated Untreated Treated Untreated Treated #1 10,100130 10,100 18 10,100 21 #2 10,100 70 10,100 3 10,100 7

As can be seen, the elements now exhibit selectivity for oxygen overnitrogen of 1.2 and 2.3, and for carbon dioxide over nitrogen of 7.2 and23. These selectivities are much lower than the solution-diffusionselectivity of the PEBAX polymer for these gas pairs, which is about 2.8for oxygen/nitrogen and about 60 for carbon dioxide/nitrogen, indicatingthat the polymer layer contains a few defects.

Nevertheless, these relatively high gas selectivities indicate that themembranes would not provide high enough flux if used as filtrationmembranes.

Example 3 Pure-Water Permeation Properties

The three membrane elements of Examples 1 and 2 were tested with purewater at a feed pressure of 60 psig, a permeate pressure of 0 psig, afeed-to-residue pressure drop of 8 psig, and a cross-flow rate of 3 gpm.The results are shown in Table 3.

TABLE 3 Membrane Element Water Flux (gfd) #1 (treated) 18 #2 (treated)14 #3 (untreated) 212

The untreated element has a pure water flux of 212 gfd. The water fluxof the treated elements has dropped by an order of magnitude or more.This confirms that the membranes are outside the preferredspecifications of the invention.

Example 4 Treated Membrane Pure-Gas Permeation Properties

Ceramic ultrafiltration membrane elements were obtained from CeraMem(Waltham, Mass.). The lab-scale elements were tubular monoliths of asilica selective layer on a recrystallized silicone carbide (RSiC)support. The elements were 1 inch in diameter, 12 inches long, and hadan effective membrane area of 0.14 m². The nominal pore size of theelements was 5 nm (50 Å).

Two elements were treated with a solution of PEBAX 1074 in n-butanol. Asingle treatment of 0.2% PEBAX was applied to Element #1205; a singletreatment of 0.1% PEBAX was applied to Element #1222. The elements wereallowed to dry and were tested as in Example 1 with pure nitrogen andcarbon dioxide. Element #1294 was left untreated and was tested forcomparison. The feed pressure was 5 psig (3 psig for Element #1294), thepermeate pressure was atmospheric, and the temperature was 22° C. Thefluxes were measured, and the permeances and the selectivities werecalculated. The results are shown in Table 4.

TABLE 4 PEBAX Membrane Concen- Permeance (gpu) Selectivity Elementtration (%) Carbon Dioxide Nitrogen CO₂/N₂ #1205 0.2 2,800 3,200 0.9#1222 0.1 8,900 9,400 0.9 #1294 — 11,700 15,300 0.8

The treated elements had lower fluxes than the untreated element. Allthree elements exhibited gas selectivity in favor of nitrogen overcarbon dioxide. The results are inconsistent with the results expectedfor a polymer selective layer separating by solution diffusion, andconsistent with separation through a finely porous membrane by Knudsondiffusion.

Example 5 Pure-Water Permeation Properties

Additional membrane elements were treated with a 0.1% solution of PEBAX1074 as in Example 4. The treated membrane elements were tested withpure water at a feed pressure of 60 psig, a permeate pressure of 0 psig,a temperature of 25° C., and a feed flow rate of 2.5 gpm. The fluxeswere measured and the permeation resistances were calculated as thefeed-to-permeate transmembrane pressure required for unit flux. Theresults are shown in Table 5.

TABLE 5 Before PEBAX After PEBAX Treatment Treatment PEBAX Water WaterMembrane Concentration Flux Resistance Flux Resistance Element (%) (gfd)(psi/gfd) (gfd) (psi/gfd) #1205 0.2 295 0.20 23 2.61 #1222 0.1 258 0.2355 1.09 #1269 0.1 — — 99 0.61 #1273 0.1 — — 92 0.65 #1275 0.1 — — 820.73

As can be seen, even a single treatment with a 0.2 wt % polymer solutionresulted in a water flux that would be too low for many applications.

Treatment with a 0.1 wt % polymer solution provided fluxes approaching100 gfd. Such a flux is sufficiently high for many applications.

The resistance is a measure of the driving force that must be applied togenerate a desired flux. As can be seen, the treatment tends to increasethe permeation resistance, although it still remains below about 1psi/gfd.

Example 6

PEBAX-Treated Element #1205 was dissected and across-sectional area wasexamined under a scanning electron microscope (SEM). FIG. 4 is an SEMphoto showing the silicone carbide support layer and the 50-μm-thicksilica selective layer of the ceramic element.

FIG. 5 is an SEM photo showing only the selective layer of the element.The PEBAX deposit is too thin to be observed at this magnification(800×). The particles on the surface are residual grit from various feedsolutions used in the permeation tests.

Example 7 Oil/Water Mixture Tests

Treated membrane Element #1275 and comparable untreated Element #1268were tested side-by-side continuously (24 hours a day) for 18 days. Thetests were performed using a bench-scale ultrafiltration test systemoperated in full recirculation mode, that is, the residue and permeatestreams exiting the elements were recombined in the feed tank andrecirculated through the test system. The initial feed solution was a2,000-ppm motor oil emulsion. The molecular weight of the motor oil wasestimated to be between 200 and 400.

To further challenge the membranes, the feed solution was changed to a5,000-ppm motor oil emulsion and the test was continued for another 7days. Throughout the 25-day test period, the feed flow rate was 7 gpmand the feed temperature was 25° C. The feed pressure was 60 psig, thefeed-to-residue pressure drop was 12 psig, and the permeate pressure wasatmospheric. The water fluxes of the elements were measured frequentlyduring the 25-day test period. The results of the tests are shown inFIG. 2.

The water flux of the untreated element immediately declined from aninitial pure water flux of 339 gfd to 214 gfd with the oil/watermixture, then continued to decline over the 18-day test period to 91gfd. The water flux of the treated element declined from an initial purewater flux of 77 gfd to 66 gfd with the oil/water mixture. The flux thenslowly increased up to 76 gfd, nearly the same as the initial pure-waterflux.

When the feed solution was changed to a 5,000-ppm motor oil emulsion,the flux of the untreated element declined to 85 gfd. The flux of thetreated element remained stable at 76 gfd.

Example 8 Synthetic Bilge Water Tests

At the end of the 25-day test period described in Example 7, bothelements were cleaned with an enzymatic cleaning solution. The feed tankwas emptied of the oil emulsion solution and refilled with a 1% solutionof Terg-A-Zyme® (Alconox, White Plains, N.Y.) in water. The test systemwas run for 24 hours at room temperature. The elements were then flushedrepeatedly with pure water to remove as much oil residue as possible,and the pure-water fluxes were remeasured. The fluxes of both elementswere unchanged.

The tests of Example 7 were then continued for another 15 days with ahighly-fouling synthetic bilge water solution containing 1,250 ppm ofoils, detergents, and solvents. As shown in FIG. 3, the water flux ofthe untreated element declined to 16 gfd within the first day andremained unchanged thereafter. The water flux of the treated elementdeclined to 38 gfd the first day, then gradually increased back up to 56gfd, where it stabilized for the remainder of the 15-day test period.

After the 15-day bilge water test (40 days total test period), theelements were again cleaned with the Terg-a-Zyme solution, this time at35° C., and flushed with pure water for three days. The pure-waterfluxes were remeasured. The flux of the untreated element increased to160 gfd; the flux of the treated element increased to 115 gfd.

The tests were continued for another seven days with a fresh batch of1,250-ppm synthetic bilge water solution. The water flux of theuntreated element declined immediately to 51 gfd and continued todecline to 33 gfd over the next seven days. The water flux of thetreated element declined to 68 gfd, then continued to decline to 51 gfdover the next seven days.

Example 9 Oil Rejection

The permeate from the test system was analyzed for total organic carbon(TOC) content to estimate the rejection properties of the membraneelements. The untreated element showed a 98+% TOC rejection; thePEBAX-treated element showed a higher TOC rejection of 99+%. Theincreased rejection is consistent with a tightening of the membrane bythe polymer treatment. The molecular weight of the motor oil wasbelieved to be in the range 200-400.

Example 10 Oil/Water Mixture Tests

Two membrane elements, a treated and a comparable untreated element, asin Example 4 were tested side-by-side continuously for 120 hours with a125-ppm oil emulsion. The tests were performed using the bench-scaleultrafiltration test system operated in full recirculation mode, thatis, the residue and permeate streams exiting the elements wererecombined in the feed tank and recirculated through the test system.These tests were conducted at a feed pressure of 60 psig and an initialpermeate pressure of 40 psig. The permeate pressure was decreased asrequired to maintain an average permeate water flux of 40 gfd. When thepermeate pressure reached atmospheric and the flux could no longer bemaintained at that level, the system was turned off, the elements werebackflushed to remove accumulated oil and other contaminants. The systemwas then put back online and the process was repeated. The performanceof the treated and untreated elements is shown in FIG. 6.

As can be seen, the resistance of the untreated element was initiallyabout 0.3 psi/gfd, and increased gradually to 2.0 psi/gfd after about 60hours. The element was backflushed, which restored the resistance toabout 1.0 psi/gfd. But, the resistance quickly increased back up toabout 1.8 psi/gfd within about 10 hours. In contrast, the resistance ofthe treated element remained constant at about 0.7 psi/gfd throughoutthe duration of the test.

Example 11 Full-Scale Element Water Permeation Tests

A full-scale ceramic ultrafiltration membrane element was purchased fromCeraMem (Waltham, Mass.). The element was the same material as thelab-scale elements, that is, a tubular monolith of a silica selectivelayer on a recrystallized silicone carbide (RSiC) support. The elementwas approximately 6 inches in diameter, 34 inches long, and had aneffective membrane area of 10.7 m². The nominal pore size was 5 nm (50Å).

A single treatment of a 0.1% solution of PEBAX® 1074 in n-butanol wasapplied and the element was allowed to dry. The treated element wasinstalled in the module housing and was tested in a full-scale testsystem with pure water. The feed pressure was 40 psig, the permeatepressure was atmospheric, and the temperature was 25° C. The feedcrossflow rate was 24 gpm, the feed-to-residue pressure drop was 8 psig,and the stage-cut was 16%. The pure-water flux was 100 kg/m²·h (60 gfd).

Based on this test, the pure-water flux at a feed pressure of 60 psigwas estimated to be 150 kg/m²·h (90 gfd), which is similar to thepure-water fluxes measured with the lab-scale elements in Example 5.

Example 12 Full-Scale Element Oil/Water Mixture Tests

A full-scale treated element similar to that used in Example 11 wastested as in Example 10, that is, the permeate pressure was decreased asneeded to maintain a constant permeate water flux. The feed pressure was60 psig, and the initial permeate pressure was about 55 psig. In thiscase, however, the permeate flux remained constant throughout theduration of the experiment (120 hours) without any reduction in thepermeate pressure. As can be seen in FIG. 7, the resistance was constantat about 0.3 psi/gfd, after an initial period of irregularity while theelement attained steady-state operation.

1. A membrane suitable for ultrafiltration, comprising an inorganicmembrane characterized by the following elements: (a) before beingtreated as described in element (b) below, the inorganic membrane has aporous feed surface with an average pore diameter in the range 10-1,000Å and is able to function as an ultrafiltration membrane; (b) theinorganic membrane has been treated by depositing an organic polymer onthe porous feed surface; (c) after being treated as described in element(b) above, the inorganic membrane continues to be able to function as anultrafiltration membrane but is unable to function as asolution-diffusion gas separation membrane, wherein the organic polymerforms a discontinuous deposit on the porous feed surface.